Fix T86210: No preview icons for non-8bit images

It looks like we never generated correct icon previews for images with
float_rects (non-8bit-images). Images from the report were 16bit pngs.

In this case, `icon_preview_startjob` would return early (it only
checked if the ImBuf `rect` was NULL -- which is the case if it has a
`rect_float` instead). This is not neccessary since `icon_copy_rect` is
perfectly capable of taking float rects.

Now correct the check and only return early if both `rect` & `rect_float`
are NULL.

note: this will not refresh icon previews from existing files
automatically. For this, use File > Data Previews > Clear Data-Block
Previews.

Maniphest Tasks: T86210

Differential Revision: https://developer.blender.org/D10601

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